1. Field of the Invention
The invention relates generally to the field of data networking. More particularly, the invention relates to asymmetric digital subscriber line technology.
2. Discussion of the Related Art
With the explosive growth of the public internet, there has been an increasing demand for subscribers to have high-speed connectivity into the public network. Such broadband access mechanisms include cable modems and digital subscriber line (DSL) technology.
The accepted standard for asynchronous digital subscriber line (ADSL) specifies the use of DMT (Discrete Multi-Tone) for the encoding process and the predominant mode of operation is the FDD (Frequency Division Duplex) mode wherein the frequency range below 130 kHz is allocated for upstream data traffic (customer premise to Network) transmission and the frequency band from 130 kHz to 1.1 MHz is allocated for downstream data traffic (Network to customer premise). Considering that subscriber loop cable is lossy and acts like a low-pass filter, the higher frequency bands experience significant attenuation. This attenuation increases with distance. As a consequence, on short loops, ADSL technology is capable of supporting of the order of 8 Mbps downstream and 800 kbps upstream. As the loop length is increased, the supportable bit-rate decreases, with the impact on the downstream bit-rate more dramatic than on the upstream bit-rate. Therefore, what is needed is an approach that reduces the affect of loop cable attenuation on bit-rate, especially downstream bit-rate.
In addition, for loops in excess of about 18 kft (typically, loop length is considered in terms of 26 AWG cable or equivalent), ADSL cannot be supported without additional equipment (Extenders) deployed in the cable plant. Therefore, what is also needed is an approach that helps to support ADSL on longer loops without deploying additional equipment in the cable plant.
Based on information made available by Service Providers, such as SBC, there are a significant number of “problem” loops in the 12 kft to 18 kft (26-AWG EWL) range. These loops are a problem because of bridged taps and/or additive noise. The model for the additive noise problem is additive white noise of between −130 dBm/Hz and −120 dBm/Hz (compared to the “usual” assumption of additive noise of approximately −140 dBm/Hz).
The service rates tariffed (proposed and/or provided) by phone companies are quite modest. At 12 kft the phone companies want to guarantee 1.5 M/128 k; at 14 kft the required rate drops to 384 k/128 k; and beyond that is “better than currently available”.
It is evident that the problems are related more to downstream capacity than upstream. Since FDD-mode DMT-ADSL uses the higher frequency band for downstream, the natural low-pass nature of subscriber loop cable introduces significantly greater attenuation for the downstream band than upstream. The lossy nature of subscriber cable indicates that bridged taps are appropriately modeled as shunt capacitors, causing a further attenuation of the downstream band (especially at the higher frequencies). The additive white noise impacts the downstream to a much higher degree than the upstream because of the (high-frequency) attenuation suffered by the signal.
One approach that has been proposed the is the use of “power boost” in the DSLAM (Digital Subscriber Line Access Multiplexer, which houses the ATU-C). The intent is to “burn through the clutter”, a phrase drawn from radar in the presence of jamming. However, the potential issues of spectral mask violations and spectral compatibility must be considered and may indeed make this approach not viable. That notwithstanding, such an approach is operationally attractive since the infrastructure is not modified, the operational issues of additional network elements is moot, and it is a “simple” matter of using different plugs in existing DSLAMs.
Another approach suggested has been to deploy additional equipment in the Central Office and the customer premise located NID (Network Interface Device; a box placed at the customer site, usually outside the building that provides a demarcation point between the Telephone Company owned subscriber loop and the customer owned inside-the-building wiring. High voltage protection is included in the NID). The CO unit would interface with the DSLAM (for ADSL) and the Class-5 Telephone Switch (for POTS); the two forms of traffic would be combined and encoded digitally onto the subscriber loop using modulation techniques like that used for G.shdsl. There would be circuitry in the NID to do the splitting of POTS and ADSL. This is an expensive way to solve the problem. Furthermore, This solution could well render the DSLAM (i.e. ADSL DSLAM) moot. The “ADSL” would exist just between the NID and the ATU-R at the CPE.
Heretofore, the requirements of reducing the affect of attenuation on the bit-rate, especially disproportionate attenuation impact on the downstream bit-rate, and avoiding the need to deploy additional equipment in the cable plant referred to above have not been fully met. What is needed is a solution that addresses these requirements.